1. Technical Field
The present invention relates to a method and apparatus for generating a Semi-compatible high definition television (SC-HDTV) signal which can be transmitted within the bandwidth of two Cable TV (CATV) channels. More particularly, a HDTV signal is formed at twice the line-scan rate of a conventional TV signal and a SC-HDTV signal is formed therefrom by transmitting a first HDTV line signal as is and a second HDTV line signal has a field differential signal quadrature amplitude modulated (QAM) on the IF or RF carrier of the first line signal.
2. Description of the Prior Art
Present day conventional television using, for example, NTSC or PAL system signals provide fairly good color pictures if the receiving conditions are good. Such pictures, however, do not come close to having the sharpness, realism and visual impact as might be found, for example, in a motion picture film or a magazine quality picture. To provide dramatically better television picture quality, experimental studies have been performed and components have been developed in an attempt to provide High-Definition Television (HDTV) pictures which approach the quality of a 35 mm color film. Such high resolution TV has been deemed especially advantageous for use, for example, in wide screen theater and home TV projection and for theatrical motion picture production and projection using magnetic tape instead of film.
Several HDTV systems have already been proposed, the parameters of which are generally disclosed in the article "The Future of High-Definition Television: First Portion of a Report of the SMPTE Study on High-Definition Television" by D. G. Fink in SMPTE Journal, Vol. 89, No. 2, February 1980 at pp. 89-94 and its conclusion in Vol. 89, No. 3, March 1980 at pp. 153-161. In the systems disclosed therein, the lines per frame range from 1023 to 2125, the aspect ratios (picture width to picture height) range from 4:3 to 8:3 and luminance bandwidths range from 20 to 50 MHz. The recommendation of this group was that a HDTV signal should be capable of a standards conversion that would provide service to NTSC, PAL and SECAM domestic services. After three years of study, however, this group concluded that a HDTV system compatible with the existing domestic services is not feasible by any means known to or envisaged by the Study Group, in view of HDTV aspect ratios and bandwidths. They further concluded that adoption of a non-compatible HDTV system for home use is problematical and would occur only after prolonged exposure to the public of HDTV projected images in theaters.
A description of one of the systems considered by the SMPTE Study Group is found in the articles "High-Definition Television System-Signal Standard and Transmission" by T. Fujio et al in SMPTE Journal, Vol. 89, No. 8, August 1980 at pp. 579-584 and "Research and Development on High-Definition Television in Japan" by K. Hayashi in SMPTE Journal, Vol. 90, No. 3, March 1981 at pp. 178-186. These articles discuss the 1125 scan line, 5:3 aspect ratio, 20 MHz luminance bandwidth system being developed in Japan. Compatibility with conventional receivers is not discussed or considered.
Other techniques labeled as either high-definition or high-resolution television system have modified the transmission of the conventional TV signal to, for example, provide increased horizontal line resolution or better luminance resolution with less objectionable subcarrier pattern. In this regard see, for example, U.S. Pat. No. 2,686,831 issued to R. B. Dome on Aug. 17, 1954 where large area flicker is allegedly canceled by subdividing the TV picture signal into three contiguous bands and transmitting each of the three bands in a certain manner during the normal sequence of a conventional picture signal. See also U.S. Pat. No. 4,296,431 issued to K. F. Holland on Oct. 20, 1981, wherein the picture signal has one of the color axes inverted at a first rate while the second color axis is inverted at a second, different, rate to provide better luminance resolution and better subcarrier pattern than allegedly found with the normal conventional signal.
Another technique for improving the horizontal and vertical definition and reducing low frequency flicker in a conventional picture is disclosed in the article "Concepts For A Compatible HIFI-Television System" by B. Wendland in NTG-Fachber (Germany), Vol. 74, September, 1980, at pp. 407-416. There is disclosed an arrangement wherein the picture quality can allegedly be improved using digital signal processing technologies. Offset sampling is used to improve horizontal definition, and a double rate stored image readout at the receiver selects the appropriate signal portions to subjectively increase the vertical definition and reduce low frequency (25 Hz) flicker. The latter three references, however, are only attempts to improve conventional television receiver picture quality and do not provide resolution comparable to a 35 mm film or magazine quality picture.
U.S. patent Appln. Ser. No. 391,973 filed for B. G. Haskell on June 24, 1982 provides a technique for generating a television signal which is capable of being converted into either a HDTV picture signal or a conventional standard picture signal for use in present day receivers. More particularly, a compatible HDTV is generated by transforming a HDTV picture signal at a first line scan rate into a color picture signal at a second line rate corresponding to the conventional TV line rate by (a) stretching each of at least two line signals by a predetermined factor, (b) forming a first line signal of at least two time stretched lines for transmission as is, and (c) forming a second line signal of the at least two time stretched lines as a line differential signal for transmission in a second portion of the compatible HDTV picture signal on a vestigial sideband carrier signal which is separated from the color subcarrier of the associated conventional TV picture signal. Such compatible HDTV signal, however, requires approximately 15-16 MHz of bandwidth for transmission and can be sent on two C-band satellite transponders or one wider K.sub.u -band satellite transponder or three present-day Cable Television (CATV) channels for local distribution.
The problem remaining in the prior art is to provide a semi-compatible HDTV picture signal which can be transmitted within the bandwidth of two CATV channels and can be converted to either a HDTV or conventional TV signal at the subscriber's receiver to make HDTV transmission reasonable for Cable TV operators.